The present invention relates generally to direct methanol fuel cells, and, more particularly, to catalysts for use with such fuel cells.
Fuel cells are an evolving source of portable electrical energy that use electrochemical reactions to generate electricity from the oxidation of a fuel source. Most fuel cell development has been directed to the use of hydrogen as the fuel source and oxygen or air as the oxidizer. However, hydrogen is not widely available as a fuel and is difficult to store in portable devices.
Conventional hydrocarbon fuels, such as gasoline and methanol, have been considered as fuels for fuel cells, typically where such liquid fuels have been reformed into a gaseous fuel stream that includes hydrogen. Reforming systems are complex, however, and reformation processes typically generate some carbon monoxide, which acts to poison conventional catalysts for the hydrogen anode reaction occurring in fuel cells. Such systems can be accommodated, but only at the expense of system efficiency and additional complications.
There have been recent developments using methanol directly as the fuel source for a fuel cell, where the xe2x80x9cexhaustxe2x80x9d is water and carbon dioxide. Methanol is widely available in liquid form and can be readily transported and stored. Moreover, methanol has a much higher energy density than hydrogen gas, which is beneficial to many potential applications of fuel cells, including automotive transportation and portable electronics.
However, conventional approaches to direct methanol fuel cells (DMFCs) using hydrogen fuel cell technology provide low fuel utilization. In both methanol and hydrogen fuel cells, a catalyst, such as platinum-ruthenium (Ptxe2x80x94Ru) for the anode and Pt for the cathode, is provided in either a supported form, where the catalyst is deposited on a carbon carrier, or unsupported form. The catalyst is applied either directly to a fuel cell membrane, typically perfluorosulfonic acid ionomer, that acts as a proton conductor or to the side of a porous electrode that contacts the membrane. The electrochemical reactions then occur at the catalyst/perfluorosulfonic acid ionomer interface.
To provide improved performance, the catalyst needs to be uniformly distributed over the membrane or gas-diffusion electrode (backing) surface and to provide a maximum surface area for contacting all the reactants. Further, the amount of catalyst (the xe2x80x9cloadingxe2x80x9d) should be minimized since the catalysts are precious metals, which are costly and relatively scarce. Prior art developments have not effectively addressed these issues.
Various aspects of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
In accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention includes homogenous suspensions (mixtures, inks) for forming anode and cathode catalysts for application to anode and cathode surfaces of a membrane for a direct methanol fuel cell. The inks comprise an unsupported or carbon-supported Pt catalyst for the cathode and an unsupported or carbon-supported Ptxe2x80x94Ru catalyst for the anode, purified water in an amount 4 to 20 times that of the catalyst by weight, and a dissolved perfluorosulfonic acid ionomer in an amount effective to provide an ionomer content in the anode and cathode catalyst layers of 20% to 80% by volume.
In another characterization of the present invention, inks are formed for use in anode and cathode catalysts for a direct methanol fuel cell. A catalyst selected from the group consisting of Pt and Ptxe2x80x94Ru unsupported or supported catalysts is mixed with purified water to form a first mixture. The first mixture is cooled to reduce evaporation of water and agitated to obtain a homogeneous suspension. A solution of perfluorosulfonic acid ionomer in alcohols is then added to the first mixture to provide a second mixture. The second mixture is agitated in the cooler to obtain a homogeneous ink suitable for application to the appropriate anode or cathode side of the membrane.
In yet another characterization of the present invention, anode and cathode inks are applied to anode and cathode sides of a membrane for a direct methanol fuel cell. An ink is formed of water, perfluorosulfonic acid ionomer, alcohols, and catalyst of unsupported or carbon-supported Ptxe2x80x94Ru for the anode or unsupported or carbon-supported Pt catalyst for the cathode while cooling and agitating the ink. The ink is placed in a cooler and continuously agitated while spraying the ink over the anode or cathode side of the membrane, anode or cathode conductive backings, or onto anode and cathode decal blanks.